1. Field of the Invention
The present invention generally relates to exhaust control valves for an engine. More specifically, the present invention relates to an exhaust control valve control arrangement that provides an exhaust valve cleaning mode which reduces the likelihood of such valves sticking as a result of use.
2. Description of Related Art
Conventional two-stroke engines generally include an exhaust port provided in each cylinder wall such that spent gases are exhausted through the exhaust port as the piston reciprocates in the cylinder. Exhaust port timing has an important effect on engine performance. Optimum exhaust port timing is dependent, in part, upon engine speed. For instance, to provide an improved engine performance, the exhaust port timing can be advanced during high-speed engine operation relative to the exhaust port timing during engine idling.
One manner of controlling the exhaust port timing is to employ exhaust control valves. Generally, these valves are of the sliding or rotating type, and do not serve to ever completely close the opening or port in each combustion chamber in two-stroke applications. Instead, each valve moves between a first position, in which the valve does not obstruct, or obstructs very little of, the exhaust port, and a second position, in which the valve partially obstructs the port. Therefore, the exhaust control valve can alter the effective cross-sectional area of the exhaust port by appearing to lower an upper surface of the exhaust port, thereby restricting the flow through the exhaust port. The movement of the valves also alters the timing of the opening and closing of the exhaust ports.
By extending the exhaust control valve into the exhaust port, it is possible to operate the engine under a higher compression ratio under low-load and low-speed conditions to improve engine performance. It is also possible to reduce the compression ratio under high-speed and high-load conditions by retracting the exhaust control valve to avoid excess pressures in the combustion chamber and pre-ignition or knocking conditions. Furthermore, closing the exhaust control valve during starting can raise the compression ratio and facilitate starting. Hence, these types of exhaust control valves are extremely effective in improving the performance of a two-stroke engine under particularly difficult running conditions without sacrificing performance under other running conditions.
There is a difficulty in conjunction with these valves however. Because the exhaust control valve is subject to exhaust gas temperatures, there must be adequate clearance provided between the valve and its surrounding structure to permit the valve to slide or rotate freely between its opened and closed positions. Of course, this clearance cannot be so great as to cause pressure leakage. In addition, because of the different materials typically employed for the exhaust control valve and the engine casting, in which the valve is mounted, compensation for differing degrees of thermal expansion is necessary. Accordingly, the clearances may actually be less when the engine is cold than when it is warm. Therefore, when the engine is started, before the engine is warmed-up, the valves are more prone to sticking.
Moreover, because these valves have a relatively small range of movement, and are not continuously moved, they may seize within the guide passage in which they are mounted. In addition, because oil may be present in the exhaust gases of two cycle engines, the oil can cause a further problem once the engine is shut off. While the engine is running, the temperature will be high enough to avoid carbonization on the valve. However, as a stopped engine cools, residual oil may carbonize, or coke, on the control valve and form deposits. These deposits, along with other foreign matter, such as sea-salt for instance, make it difficult to operate the valve when the engine is restarted. In short, these deposits tend to inhibit a smooth operation of the valve. Of course, the aforementioned differential thermal expansions will also further aggravate this situation.